The present invention relates to bulk diffusers and, more specifically, to projection television screens which incorporate bulk diffusers therein to control, primarily, the vertical dispersion of the projected light.
Bulk diffusers may be described as including minute spheres embedded in a substrate of a slightly different index of refraction. The spreading characteristics of bulk diffusers are a function of the size and index of refraction of the spheres, the thickness and index of refraction of the substrate, and the loading density of the spheres in the substrate.
Bulk diffusers used in projection television screens are characteristically specified by their intensity half-angle. The intensity half-angle is defined as the angle at which the intensity falls to half the maximum intensity. It has been generally presumed that given a specific intensity half-angle, the intensity profile would be Gaussian. However, this could not explain the appearance of some of the artifacts seen on projection television screens, namely, (1) the picture is visible at much larger angles than the measured intensity half-angle, (2) the solution of the screen is not as high as predicted, and (3) the screen has a lower gain than that predicted.
In a typical projection television screen, the bulk diffuser is specified to be 3 mm. thick and have an 8.degree. intensity half-angle, it being assumed that the intensity profile would be Gaussian. In this typical bulk diffuser, the refractive indices of the spheres and the substrate are 1.53 and 1.49, respectively, the volume ratio of the spheres being equal to 2%, and the spheres having a range in diameters from5 .mu.m. to 100 .mu.m.
In the article "Diffraction Analysis of Bulk Diffusers for Projection Screen Applications", appearing in the Journal of The Optical Society A, December, 1985, applicants disclosed a computer program for analyzing bulk diffusers which assumes that all the spheres have the same diameter. Using this program, the measured intensity half-angle of 8.0.degree., and the range of diameters 5-100 .mu.m., an effective sphere diameter is determined as equally 17.3 .mu.m. (The effective sphere diameter being the single diameter which accurately, within reason, represents the range in diameter of the spheres.) The program may then be used to plot the predicted intensity as a function of angle for this diffuser. This plot is shown in FIG. 1 along with the measured intensity for a sample bulk diffuser. By comparing the predicted and measured results to the curve corresponding to a Gaussian profile, it is clear that the predicted and measured profiles are not Gaussian.